1. Field of the Invention
The invention relates to a doctor blade for use in a screen printing machine or for handling by an operator with a view to printing or transferring fluids or pastes, or other applications.
2. Description of the Prior Art
Doctoring heads of the type comprising a doctor blade carrier and a deformable doctor blade extending in the longitudinal direction and being rigidly associated to each other are known. Such known doctoring heads constitute the subject-matter of many various embodiments. They are fixed to carrier means of the machine by means of screws, bows or other clamping or fixing means.
The pressure to be exerted downwardly must simultaneously:
eliminate what is called in the art "off contact", i.e. the spacing of 2 to 5 millimeters existing between the screen and the printing carrier, this function essentially lying with the ends of the doctor blade; PA1 compensate for thickness differences of the stencil, the carrier of the image applied on or under the screen; PA1 compensate for irregularities of the printing carriers whose thickness may vary more or less than 6% for cardboard, for instance, depending on the seams and hems for textile printing, etc.; PA1 drive an appropriate and constant ink portion through the screen for carrying out the printing process, releasing energy in dependence upon the ink thixotropy; PA1 push the ink wave accumulating before the doctor blade according to its feeding rate. PA1 eliminate the "off contact"; PA1 drive an adequate and constant part of the ink throught the screen to carry out the printing operation; PA1 push the ink wave accumulating before the doctor blade according to its feeding rate. PA1 compensate for irregularities of the carrier of the image applied on or under the screen; PA1 compensate for irregularities of the printing carriers; PA1 compensate for the surface irregularities of the printing tables.
Before each doctoring action, a metallic counterdoctor or, according to the terminology in the art, a "flood bar" slightly touches the screen in order to respread the ink which had been pushed towards an edge of the screen by the preceding printing process.
The doctor blade traditionally consists of rubber and typically has the shape of a rectangular parallelepiped, continuous both in the longitudinal and transverse directions. It may also comprise, in the portion in contact with the screen, different profiles, which may be considered more favourable for certain work.
The doctor blade must simultaneously comprise features which seem to be contradictory.
It must be sufficiently rigid (tough) in order to:
Moreover, the doctor blade must be sufficiently resilient in order to:
According to other prior art embodiments, the doctor blades are incorporated in machines. More particularly, reference may be made to U.S. Pat. No. 4,854,230, EP-A-140 165, EP-A-460 267. Irrespective of whether the doctor blade is intended for manual function, as taught in GB-A-2 175 850, U.S. Pat. No. 4,102,266, FR-A-2 302 199, U.S. Pat. No. 4,989,511, or whether it is incorporated in a machine, there arises a problem to have a doctor blade comprising both the rigidity and the flexibility required, which had been explained above.
Document U.S. Pat. No. 5,345,862 provides in one embodiment a doctor blade comprising two external layers and a more rigid internal layer.
Reference may also be made to EP-A-0298911 and DE-B-38 12 826.
EP-A-167 906 provides a carrier bar of a hard-elastic material inserted in the doctor blade carrier, to which a profiled blade of a resilient material is connected.
Irrespective of the realization variants envisioned for the doctor blade, there arise a certain number of problems for which no satisfying solution has hitherto been found yet.
The doctoring carrier inevitably comprises surface irregularities. During doctoring, the movement of the doctor blade causes vibrations due to these surface irregularities, which both impair the printing quality and contribute to the wear of the machine comprising the doctor blade.
In addition to the vibrations, the surface irregularities influence the printing process since the operating conditions, more particularly the contact pressure of the doctor blade, its inclination angle and even its contact with the printing carrier are variable from one location to the other of the printing surface.
If the doctoring speed is high, the accumulated ink before the doctor blade may result in a phenomenon of "encreplanage; ink-planing" (neologism in analogy with aquaplaning and meaning that the doctor blade is lifted off by an ink film) which again impairs printing quality.
Known doctor blades cannot be conveniently regulated with respect to their stiffness, both in time and locally in any zone of their length.
It is certainly known to provide a doctor blade at its end portions with lowering means, screens as described in U.S. Pat. No. 5,345,862, in particular. Also it has been suggested to provide doctor blades with a curved profile. However, all these implementations are complicated, often not very precise, of limited applicational scope and, finally, of unsatisfactory efficacy with respect to the current requirements relating to quality and productivity.
Thus, it is the object of the invention to overcome the drawbacks or limitations of the current doctoring heads.
More particularly, the invention aims to resolve the problems relating to surface irregularities of the doctoring carriers.